As telecommunications networks continue to expand their need for bandwidth, it is becoming increasingly necessary to introduce new technologies to keep up with growing demands. These technologies must not only facilitate the need for bandwidth, but be easily incorporated into today's network infrastructure. At the same time, they must expand to fit the requirements of the future. While current telecommunication systems comprise a combination of electronic and optical data-transmission, there is pressure to move toward all-optical networks due to the increased bandwidth provided by high bit-rates and parallel transmission through wavelength division multiplexing.
Currently, optical networks use light for much of the transmission of data between nodes in an optical circuit. Optical cross-connects function as switches in these nodes; routing signals arriving at one input-port to one of a variety of output-ports. Currently, the majority of optical cross-connect systems consist of high-speed electronic cores, which are complex, cumbersome, and expensive. These switches typically require the light signal to be translated into an electronic signal, which is switched or routed to an output-port before being reconverted to light. The complexity, size, and expense of such optical-to-electronic-to-optical (OEO) components become even more problematic with higher bit-rates and port counts, even as the cost of electronic components decreases, due to cross-talk and RF transport issues.
OEO devices are typically the rate-limiting component in any optical network. As such, many options are being considered to reduce the need for both OEO conversions, as well as electronic-signal processing in optical network components. This has lead to emphasis being placed on the development of “all-optical” switching technology, in which optical signals passing through a switch are diverted to the appropriate destination without being converted to electrons.
For most current applications, electronically controlled optical cross-connects with optical-cores can be used as an all-optical switch. In these devices, light routing does not require OEO conversion, but operation of the switch is electronic. The various all-optical switching technologies that currently support such systems include electromechanical switches (MEMS or bulk optics), thermo-optic switches (phase shift, capillary, or “bubble”), and electro-optic switches (LiNbO3, liquid crystal). In addition, a variety of nonlinear optical switches (e.g., semiconductor optical amplifiers) use a light beam, rather than electronics, to operate the switch.
Many all-optical switching technologies are relatively slow, and are therefore only appropriate for static configuration control. Applications such as basic fiber/wavelength routing, provisioning and restoration typically require switching speeds around 1 ms. These slow all-optical switches, however, are inadequate for fast switching applications such as dynamic packet switching (˜1 ns), header reading in packet switched networks (<25 ps), and all-optical data-processing (<1 ps).
Currently, only devices based on nonresonant nonlinear optical phenomena, such as cross-gain modulation (XGM) in semiconductor optical amplifiers, χ(2) based pheonomena [e.g., difference-frequency mixing (DFM)], χ(3) (or Kerr) based phenomena [e.g., cross-phase modulation (XPM) and four-wave mixing (FWM)], have the potential to switch at rates required for packet-switching, optical data processing and other high-speed switching applications. Such devices have the potential for switching speeds approaching (and even exceeding) ten terabits per second (10 Thit/s), or 10 trillion bits per second.
Research involving χ(3)-based all-optical switching has been pursued since the mid-1980s, when the development of ultrashort laser sources enabled the widespread study of χ(3) nonlinear optical phenomena in a variety of materials. This research has primarily focused on all-optical switching in silica fiber-based devices. This is due to the relatively large figure-of-merit (FOM) of silica. There are many practical definitions of a FOM that takes into account many parameters that are important and relevant to all-optical switching. One example of such a FOM is defined as             Δ      ⁢                          ⁢      n              α      ·      τ        ,where Δn is the induced refractive index change, α is the linear and nonlinear absorption coefficient, and τ is the response time of the material. In general, the larger the FOM, the better will be the performance of the all-optical switching. Another definition of an important FOM is 2γ/βλ, where γ is the nonlinear index of refraction, β is the two-photon absorption coefficient, and λ is the wavelength of operation. In this case, useful all-optical switching occurs when FOM>1. Due to the extremely low linear and nonlinear losses of light at telecommunications wavelengths in silica, the FOM of silica is large even though Δn and γ (which are related to Re[χ(3)]) are small.
Many all-optical switching devices using silica fiber have been demonstrated (e.g. nonlinear directional couplers, nonlinear optical loop mirrors, and soliton-based switches). Due to the small γ of silica, however, impractical fiber lengths (˜1000 km) are required for these devices to operate at typical telecommunication powers (˜10 mW). As a result, there is a great deal of interest in developing materials with both a large FOM and a large γ to reduce overall device sizes and latency. Device sizes ˜1 mm or less are desirable for integration of multiple devices and to provide insensitivity to temperature fluctuations and manufacturing fluctuations (i.e. tight tolerance over long distances). In addition, low latency is needed as the data rates increase.
In addition to large nonlinearities, it is critical that commercial optical switching components are low cost and compatible with high-throughput automated fabrication. Historically, semiconductor processing, used to make microprocessor chips, has been one of the most cost-effective and automated processes for miniaturization. While this technology is extremely advanced in the field of microelectronics, it is still in its infancy with respect to optics. While waveguide structures have been fabricated using these techniques, they are rarely automated due to the incompatibility of the available active materials. For instance, crystalline LiNiO3 cannot be arbitrarily inserted within a waveguide created by these techniques. In addition, polymeric nonlinear materials, which are more easily processed, typically have χ(3) that is too low for efficient switching.
Presently, there is a variety of approaches being pursued to reduce the size of χ(3)-based all-optical switches. Approaches being considered include using semiconductor optical amplifiers (SOAs); manufacturing photonic bandgap structures with nonlinear materials; enhancing nonresonant optical nonlinearities using local field effects; and developing new crystalline materials and polymeric materials with high optical nonlinearities.
While proof-of-concept for all-optical switches based on SOAs has been shown, problems with amplified spontaneous emission buildup currently make cascading many of these switches problematic. In addition, the materials used for SOAs (typically InP) are expensive and create inherent difficulties with coupling to standard silica fibers and waveguides. Photonic bandgap materials are another promising approach, but manufacturing is still beyond current practical capabilities. While enhancing nonlinearities using local field effects is an interesting approach, enhancement factors of only ˜10× have been achieved to date. Finally, new nonlinear crystalline materials have been developed (e.g. periodically poled LiNbO3), but are typically expensive, and difficult to process, making incorporation into waveguide devices problematic. Nonlinear polymers, with more appealing mechanical properties, have also been developed, but problems such as kinks in the polymer chains limit the maximum nonlinearity to a value still unsuitable for practical all-optical applications. In cases where highly nonlinear polymers have been produced (e.g. polyacetylene), many of the appealing mechanical properties are lost, creating problems similar to those found in crystalline materials.
In addition to high nonlinearity and processability, nonlinear materials must also be low-loss in the wavelength range-of-interest (e.g. from absorption or scattering). They must also have a linear index of refraction that is compatible with the specific architecture of the device in which they are to be used (e.g. a nonlinear waveguide core must have an index of refraction higher than the cladding surrounding it). As such, it has been extremely difficult to find a practical material that simultaneously satisfies all of the requirements for a commercial χ(3)-based nonlinear device.
The ideal χ(3)-based nonlinear optical material will have the following characteristics:                1. Large Re[χ(3)] in the wavelength range-of-interest (Re[χ(3)] is directly related to Δn and γ).        2. Extremely low optical losses from single- and multi-photon absorption, and/or resonant and nonresonant scattering in the wavelength range-of-interest.        3. A multiphoton transition near the wavelength range-of-interest (for resonant and near resonant enhancement of χ(3)).        4. A precisely selected linear index of refraction, compatible with waveguides of the intended device architecture.        5. Physical and chemical compatibility with the specific device architecture and materials with which it will be used.        6. The ability to be processed for incorporation into optical devices.        7. Low cost.        
While many materials may have one or more of these desirable characteristics, at present, no single material comprises all of the optical and physical properties required for an optimal χ(3)-based optical switch. In fact, besides SOAs, no commercial devices are currently available, primarily due to a lack of appropriate nonlinear optical materials. The current invention provides a solution to this problem by providing a novel nonlinear material, as well as structures, devices and applications enabled by this material.